Atomically Dispersed Palladium Driving Reductive Catalytic Fractionation of Lignocellulose into Alkene-Functionalized Phenols

As lignin represents the only renewable aromatic organic resource in nature, the search for efficient catalysts for lignin depolymerization to produce functionalized phenols with high yields and selectivity is of great importance but challenging. Herein, we report an extremely low-loaded and atomically dispersed Pd catalyst anchored on a carbon carrier (Pd/ZnO/C) for the reductive catalytic fractionation (RCF) of lignocellulose, from which nearly theoretical maximum aromatic monomer yield (40.7 wt %), good selectivity to 4-n-propenyl-substituted phenols (62 mol %), and high retention of cellulose and hemicellulose (92 and 70 wt %) were realized. The calculated turnover number (TON) (375 (molaromatics) mol(Pd -)1) is 46 times higher than that of the commercial Pd/C catalyst. The Pd/ZnO/C catalyst possessed stability and reusability in recycled RCF experiments. The plausible reaction pathway of the current catalytic system was proposed based on the detailed investigation of a series of lignin model compounds. Using 4-n-propenyl guaiacol from Pd/ZnO/C-catalyzed RCF of Chinese fir (Cunninghamia lanceolata) as the substrate, the preparation of the symmetrical monomeric stilbene (a natural product) was also achieved. This study provides a feasible strategy for the depolymerization of lignin into alkene-functionalized phenols with low cost and high efficiency.